1. Field of the Invention
This invention relates to an improved regeneration procedure for sulfur-contaminated molecular-sieve catalysts used in the conversion of hydrocarbons, and more specifically for zeolite-containing catalysts used in the catalytic reforming of gasoline-range hydrocarbons.
2. General Background
The catalytic reforming of hydrocarbon feedstocks in the gasoline range is an important commercial process, practiced in nearly every significant petroleum refinery in the world to produce aromatic intermediates for the petrochemical industry or gasoline components with high resistance to engine knock. Demand for aromatics is growing more rapidly than the supply of feedstocks for aromatics production. Moreover, the widespread removal of lead antiknock additive from gasoline and the rising demands of highperformance internal-combustion engines are increasing the required knock resistance of the gasoline component as measured by gasoline "octane" number. The catalytic reforming unit therefore has been required to operate more efficiently at higher severity in order to meet these increasing aromatics and gasoline-octane needs. This trend has created a need for more effective reforming processes and catalysts.
Catalytic reforming generally is applied to a feedstock rich in paraffinic and naphthenic hydrocarbons and is effected through diverse reactions: dehydrogenation of naphthenes to aromatics, dehydrocyclization of paraffins, isomerization of paraffins and naphthenes, dealkylation of alkylaromatics, hydrocracking of paraffins to light hydrocarbons, and formation of coke which is deposited on the catalyst. Increased need for aromatics and gasoline octane have turned attention to the paraffin-dehydrocyclization reaction, which is less favored thermodynamically and kinetically in conventional reforming than other aromatization reactions. Considerable leverage exists for increasing desired product yields from catalytic reforming by promoting the dehydrocyclization reaction over the competing hydrocracking reaction while minimizing the formation of coke.
The effectiveness of reforming catalysts comprising a non-acidic L-zeolite and a platinum-group metal for dehydrocyclization of paraffins is well known in the art. The use of these reforming catalysts to produce aromatics from paraffinic raffinates as well as naphthas has been disclosed. The increased sensitivity of these selective catalysts to sulfur in the feed also is known. Nevertheless, this dehydrocyclization technology has not been commercialized during the intense and lengthy development period. The extreme catalyst sulfur sensitivity of current reforming catalysts selective for dehydrocyclization, providing surprising results when using novel methods for maintaining the performance of these catalysts such as the regeneration procedure of the present invention, is only now being recognized.